Language activation studies with positron emission tomography

Behavioural tasks produce changes in regional cerebral blood flow (rCBF), the result of increased local neural activity. These changes can be measured with positron emission tomography (PET). Language activation studies by means of PET are being used to relate regional patterns of cerebral activation to information-processing models of speech and reading. Significant activation confined to both superior temporal gyri has been observed when normal subjects hear words played backwards, listen to non-words, and perform category judgements on pairs of heard real words. Prestriate cortex is activated by seeing strings of letter-like symbols, consonant strings, pronounceable non-words and real words, with additional activation in left medial prestriate cortex in response to the non-words and real words. Left posterior superior temporal gyrus (PSTG), left dorsolateral prefrontal cortex (DLPFC) and supplementary motor area (SMA) are engaged when subjects retrieve verbs from memory to match nouns. Finally, primary sensorimotor cortex is activated during articulation. There is particular interest at present in the precise roles of left PSTG and DLPFC in single-word comprehension and generation, and interpretation of the results depends critically on the design of the single-word tasks used for behavioural activation.

Emotion-perception interplay in the visual cortex: "the eyes follow the heart"

Emotive aspects of stimuli have been shown to modulate perceptual thresholds. Lately, studies using functional Magnetic Resonance Imaging (fMRI) showed that emotive aspects of visual stimuli activated not only canonical limbic regions, but also sensory areas in the cerebral cortex. However, it is still arguable to what extent such emotive, related activation in sensory areas of the cortex are affected by physical characteristic or attribute difference of stimuli. To manipulate valence of stimuli while keeping visual features largely unchanged, we took advantage of the Expressional Transfiguration (ET) of faces. In addition, to explore the sensitivity of high level visual regions, we compared repeated with unrepeated (i.e. different) stimuli presentations (fMR adaptation). Thus, the dynamics of brain responses was determined according to the relative signal reduction during "repeated" relative to "different" presentations ("adaptation ratio"). Our results showed, for the first time, that emotional valence produced significant differences in fMR adaptation, but not in overall levels of activation of lateral occipital complex (LOC). We then asked whether this emotion modulation on sensory cortex could be related to previous personal experience that attached negative attributes of stimuli. To clarify this, we investigated Posttraumatic Stress Disorder (PTSD) and non-PTSD veterans. PTSD is characterized by recurrent revival of trauma-related sensations. Such phenomena have been attributed to a disturbed processing of trauma-related stimuli, either at the perceptual level or at the cognitive level. We assumed that PTSD veterans would differ from non-PTSD veterans (who have similar combat experience) in their high order visual cortex responses to combat-related visual stimuli that are associated with their traumatic experience. An fMRI study measured the cerebral activation of subjects while viewing pictures with and without combat content, in "repeated" or "different" presentation conditions. The emotive effect on the visual cortex was found, again, only in the fMR-adaptation paradigm. Visual cortical regions showed significant differences between PTSD and non-PTSD veterans only in "repeated" presentations of trauma-related stimuli (i.e. combat). In these regions, PTSD veterans showed less decrease in signal with repeated presentations of the same combat-related stimuli. This finding points to the possibility that traumatic experience modulates brain activity at the level of sensory cortex itself.

Feeling or features: different sensitivity to emotion in high-order visual cortex and amygdala

Emotionally loaded visual stimuli have shown increased activation in visual and cortex limbic areas. However, differences in visual features of such images could confound these findings. In order to manipulate valence of stimuli while keeping visual features largely unchanged, we took advantage of an "expressional transfiguration" (ET) effect of faces. In addition, we used repetition effects, which enabled us to test more incisively the impact of the ET effect. Using the ET manipulation, we have shown that the activation in lateral occipital complex (LOC) was unaffected by valence attributes, but produced significant modulation of fMR adaptation. Contrary to LOC, amygdala activation was increased by ET manipulation unrelated to the adaptation. A correlation between amygdala and LOC adaptation points to a possible modulatory role of the amygdala upon visual cortex short-term plasticity.

Gesture and the processing of speech: neuropsychological evidence

Patterns of speech-related ('coverbal') gestures were investigated in three groups of right-handed, brain-damaged patients and in matched controls. One group had anomic aphasia with a primarily semantic impairment ('semantic'); one group had a primarily phonological impairment, reflected in both repetition and naming ('phonologic'); a third group had a primarily conceptual impairment, with relatively good naming ('conceptual'). Coverbal gestures were video recorded during the description of complex pictures and analyzed for physical properties, timing in relation to speech and ideational content. The semantic and phonologic subjects produced a large number of ideational gestures relative to their lexical production, while the related production of the conceptual subjects was similar to that of the unimpaired controls. The composition of ideational gestures in the semantic and phonologic groups was similar to that of the control groups, while conceptual subjects produced fewer iconic gestures (i.e., gestures that show in their form the content of a word or phrase). The iconic gestures of the conceptual patients tended to start further from their lexical affiliates than those of all other subjects. We conclude that ideational gestures probably facilitate word retrieval, as well as reflect the transfer of information between propositional and non-propositional (visual and motoric) representations during message construction. We suggest that conceptual and lexical processes differ in the way they constrain ideational gestures.

Noun and verb retrieval by normal subjects. Studies with PET

PET activation studies identify significant local changes in regional cerebral blood flow (rCBF) in contrasts of behavioural tasks with control states, and these local changes identify net changes in local synaptic activity. A number of studies on word retrieval have all demonstrated left frontal (dorsolateral and medial) involvement in the task. However, there have been differences in the responses observed in the left temporal lobe, with variously a deactivation (significant decrease in rCBF), no response and an activation (significant increase in rCBF). In the four studies described here, we have examined word (verbs and nouns) retrieval contrasted with a number of different control states. The studies confirmed extensive activation of the left dorsolateral prefrontal cortex and, medially, the anterior cingulate cortex and the supplementary motor area (SMA). Activations of the left posterior temporal lobe and the inferior parietal lobe were consistently demonstrated when word retrieval was contrasted with a rest state. Contrasts with other single word tasks controlled out the activation in the perisylvian part of the left posterior temporal lobe, suggesting a role for this region in lexical processing. The left inferolateral temporal cortex and the posterior part of the inferior parietal lobe were only activated by word retrieval, particularly verbs. It is proposed that these activated regions reflect access to semantic fields.

Cognition in stroke

INTRODUCTION

Despite the current interest in criteria for vascular dementia, global, as opposed to focal, cognitive change after cerebral infarction has rarely been studied.

MATERIAL AND METHODS

We documented the neuropsychological changes one to three weeks and three months post infarct in 25 unselected patients with acute, first cerebral infarcts.

RESULTS

Improvements were seen in processes through to have a large subcortical component and in those mediated in the right hemisphere. Memory was relatively lightly affected.

CONCLUSION

The minimal deficits seen in memory and the predominance of subcortical changes are at variance with the currently suggested criteria for vascular dementia. Further data of this kind are needed before firm criteria can be proposed for the global pattern of cognitive changes expected in vascular dementia.

Is parkinsonian arm tremor a resting tremor?

Arm tremor in Parkinson's disease occurs during rest which has been explained by a specific tremulous mechanism activated during rest. During action or posture maintenance, the tremor may persist or recur, but with reduced amplitude. We present 2 patients with idiopathic Parkinson's disease who showed persisting arm tremor which was most marked during action rather than during rest. The patterns of upper limb tremor were investigated during rest, maintenance of fixed posture, and slow movement, using an advanced computerized movement-monitoring system (Coda 3). Four parkinsonian patients showing tremor under all conditions were studied, comparing 2 patients whose tremor was most marked during rest with 2 whose tremor was most marked during action. All showed intermediate amplitudes during posture maintenance. Patterns of tremor frequency were very similar between patients, irrespective of amplitude characteristics. These features suggest that tremors in all patients originate in a single, complex mechanism which may involve both central and peripheral mechanisms.

Crossed anomic aphasia: mild naming deficits following right brain damage in a dextral patient

A detailed case study is reported of crossed aphasia (CA) in a dextral patient, bearing upon such controversial issues as intrahemispheric localisation of language function and hemispheric reversal of nonverbal function. DA, a man aged 37, developed a mild naming problem due to right temporal lobe haematoma. Apart from a mild acquired stutter, his continuous speech was fluent and had a normal proportion of open to closed class lexical items. His naming deficit appears to originate in the 'blocking' or 'disconnection' of the phonological lexicon: he could usually give a functional definition of un-named items and retrieve them with the help of a phonemic cue. Lexical retrieval appears his only language deficit, as he had no comprehension or phonological discrimination deficits. DA showed no visuo-spatial or auditory-nonverbal deficits, suggesting the complete reversal of hemispheric specialisation.

Distribution of cortical neural networks involved in word comprehension and word retrieval

Six normal volunteers were studied with positron emission tomography to identify the cortical neural networks that participate in the processing of single words. Activity-related changes in regional cerebral blood flow were measured consecutively on 6 occasions in each subject, 2 while the subject was at rest and 4 while single word language tasks were being performed. The data from each subject were standardized for brain shape and size, reconstructed parallel to the intercommissural line, normalized for global flow differences, and then averaged for each activation condition across the 6 subjects. Significant areas of change in rCBF (P less than 0.05, with appropriate Bonferroni corrections) between task and rest conditions were displayed with reference to the coordinates of a standard neuroanatomical atlas. We have demonstrated that categorical judgements on heard pairs of real words activate neural networks along both superior temporal gyri, but with an anatomical distribution no different from that seen when the subjects listened to nonwords: the tasks would appear to be very different in cognitive demands but not in terms of the distribution of activation. However, during a verb generation task that involved thinking of verbs appropriate to heard nouns presented at a slow rate, the only temporal region activated was the left posterior superior temporal association cortex (Wernicke's area). Further analysis showed that whereas activation in other superior temporal regions, both left and right, correlated with rates of word presentation during the 4 tasks, there was no such correlation in Wernicke's area; evidence that this site is responsible for more than early acoustic processing. During verb generation there was also activation of left premotor and prefrontal cortex (including Broca's area and the supplementary motor area). The supplementary motor area is thought to be involved in the motor planning of speech. The subjects did not vocalize during the task, and therefore it would appear that the act of retrieving words from semantic memory activates networks concerned with the production of speech sounds. We conclude that single word comprehension and retrieval activate very different distributed regions of cerebral cortex, with Wernicke's area the only region engaged by both processes and with participation during silent word generation of networks involved in vocalization.

Distribution of cortical neural networks involved in word comprehension and word retrieval

Six normal volunteers were studied with positron emission tomography to identify the cortical neural networks that participate in the processing of single words. Activity-related changes in regional cerebral blood flow were measured consecutively on 6 occasions in each subject, 2 while the subject was at rest and 4 while single word language tasks were being performed. The data from each subject were standardized for brain shape and size, reconstructed parallel to the intercommissural line, normalized for global flow differences, and then averaged for each activation condition across the 6 subjects. Significant areas of change in rCBF (P less than 0.05, with appropriate Bonferroni corrections) between task and rest conditions were displayed with reference to the coordinates of a standard neuroanatomical atlas. We have demonstrated that categorical judgements on heard pairs of real words activate neural networks along both superior temporal gyri, but with an anatomical distribution no different from that seen when the subjects listened to nonwords: the tasks would appear to be very different in cognitive demands but not in terms of the distribution of activation. However, during a verb generation task that involved thinking of verbs appropriate to heard nouns presented at a slow rate, the only temporal region activated was the left posterior superior temporal association cortex (Wernicke's area). Further analysis showed that whereas activation in other superior temporal regions, both left and right, correlated with rates of word presentation during the 4 tasks, there was no such correlation in Wernicke's area; evidence that this site is responsible for more than early acoustic processing. During verb generation there was also activation of left premotor and prefrontal cortex (including Broca's area and the supplementary motor area). The supplementary motor area is thought to be involved in the motor planning of speech. The subjects did not vocalize during the task, and therefore it would appear that the act of retrieving words from semantic memory activates networks concerned with the production of speech sounds. We conclude that single word comprehension and retrieval activate very different distributed regions of cerebral cortex, with Wernicke's area the only region engaged by both processes and with participation during silent word generation of networks involved in vocalization.

Neuropsychological assessment of cognitive change in dementia

The problem of long-term assessment of cognitive change in dementia is defined as that of avoiding floor and ceiling effects in testing. Unlike diagnosis, the establishment of norms is viewed as secondary. This perspective allows the formulation of strategic guidlines in devising test batteries for longitudinal studies of cognitive change in dementia. A number of tests complying with these guidelines are described, tapping a wide range of cognitive abilities such as semantic analysis of verbal and visual material, semantic memory, object recognition, visuoconstruction and serial planning.

Gesture, speech, and computational stages: a reply to McNeill

McNeill (1985) claimed that speech and its accompanying gestures "share a common computational stage" that is located early in the speech production process. This position depends on two crucial arguments that we believe are unsound: first, that gestures "are synchronized with linguistic units in speech" and, second, that gestures "have semantic and pragmatic functions that parallel those of speech." Careful analysis of the relevant data indicated that although the processes underlying gestural production are globally autonomous, they are linked to speech production at more than just one computational stage. An account is offered that explicates these links.

The disconnection in anomic aphasia between semantic and phonological lexicons

PB, a right handed woman aged 26, has developed anomic language disturbances following right frontal and left temporal brain damage due to a road traffic accident. Her anomia shows exceptional features: it occurs in naming tasks in test conditions but not in continuous speech; it involves comprehension deficits but very few semantic errors in expression; it improves with phonemic, but not with semantic cues. These features, it is argued, indicate the disconnection between two intact lexicons: the semantic and the phonological.

The relationship between head movements and speech dysfluencies

In four subjects, the correlations between dysfluencies in spontaneous speech and head movements, monitored by polarized-light goniometry, were studied in order to establish accurately their relative timing and the interaction between their linguistic and physical properties. Speech dysfluencies were categorized according to grammatical and physical properties, while head movements were classed as previously according to kinematic properties. Distinct head movement occurred after most dysfluencies, especially in association with large increases in loudness, showing significant correlations between movement classes and dysfluency categories. For instance, wide, linear movements ("postural shifts" or PS) tended to occur after "grammatical" pauses (between clauses or sentences); narrow, rapid movements (RM) tended to occur after dysfluencies inside grammatical boundaries, especially after short pauses. Findings were interpreted as consistent with previous speculations, assigning head movement a role in speech production, especially in dissipating energy and helping to co-ordinate movements of the articulators.

Head movement correlates of juncture and stress at sentence level

Body movement during speech has been recognized as closely relating to suprasegmental features, but little evidence has been offered to support this thesis, probably for lack of adequate techniques. The present study investigated this issue by continuously recording, with a polarized-light goniometer, movement of the head in four subjects engaged in conversation. Rapid movements were found to indicate stress, while juncture involved contrasting ordinary movements with stillness. This was believed to indicate that the dissipative structure coordinating speech resorts to body movement in regulating high energies, and that prosodic features may accentuate inner continuity by varying smoothly towards terminal juncture.

Loss of visual constancy of a fixated target induced by rapid movement of the head

The possibility that various types of conjugate eye movements are not merely a means of efficient perception but also determine the related visual effects is tested by imposing rapid (saccadic) movements upon the (slow) compensatory mechanism. 10 subjects were asked to shake and nod the head, at separate times, with increasing speed while fixating a stationary target. As the head reaches a frequency of resonance, at around 5 Hz, the fixated target appears to be jumping counter-head-wise.